Sondra Locke, 38 years old at the time of filming, plays Rosemary Clooney from ages 17 to 40.
Sondra Locke and Rosemary Clooney both died of cancer at age 74.
The real Rosemary Clooney (1928-2002) was only sixteen years older than Sondra Locke (1944-2018). It's unknown whether Clooney was ever made privy to this, since Locke had always lied about her age, and would continue to masquerade about being younger for the rest of her life.
Tony Orlando and Sondra Locke were the same age, whereas the real José Ferrer was 16 years older than Rosemary Clooney.